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Summary of Significant Accounting Policies Earnings Per Share (Details) (USD $)
Share data in Thousands, except Per Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]      
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount 0us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 0us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 2,102us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Earnings Per Share Reconciliation [Abstract]      
Net Income (loss) $ 666,051,000us-gaap_ProfitLoss $ 170,935,000us-gaap_ProfitLoss $ (54,249,000)us-gaap_ProfitLoss
Weighted Average Number of Shares Outstanding, Basic 67,230us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 66,615us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 65,138us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Incremental Common Shares Attributable to Share-based Payment Arrangements 814us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 1,383us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 0us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements
Incremental Common Shares Attributable to Conversion of Debt Securities 0us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities [1] 0us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities [1] 0us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities [1]
Weighted Average Number of Shares Outstanding, Diluted 68,044us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 67,998us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 65,138us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Basic net income (loss) per common share $ 9.91us-gaap_EarningsPerShareBasic $ 2.57us-gaap_EarningsPerShareBasic $ (0.83)us-gaap_EarningsPerShareBasic
Diluted net income (loss) per common share $ 9.79us-gaap_EarningsPerShareDiluted $ 2.51us-gaap_EarningsPerShareDiluted $ (0.83)us-gaap_EarningsPerShareDiluted
Convertible Debt [Member]      
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]      
Debt Instrument, Principal Amount Denominator for Conversion into Common Stock     1,000sm_DebtInstrumentPrincipalAmountDenominatorForConversionIntoCommonStock
/ us-gaap_DebtInstrumentAxis
= us-gaap_ConvertibleDebtMember
Debt Instrument, Convertible, Conversion Price     $ 54.42us-gaap_DebtInstrumentConvertibleConversionPrice1
/ us-gaap_DebtInstrumentAxis
= us-gaap_ConvertibleDebtMember
Retained Earnings      
Earnings Per Share Reconciliation [Abstract]      
Net Income (loss) $ 666,051,000us-gaap_ProfitLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember		 $ 170,935,000us-gaap_ProfitLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember		 $ (54,249,000)us-gaap_ProfitLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
[1] (1) For the year ended December 31, 2012, the shares into which the 3.50% Senior Convertible Notes were convertible were anti-dilutive and excluded from the calculation of diluted earnings per share.